1. Field of the Invention
The present invention relates to a stem, viz. a header for a semiconductor device and in particular to a stem for a semiconductor device on which discrete semiconductor chips or pellets are mounted.
2. Description of the Prior Art
In stems for a semiconductor device in the prior art, as shown in FIGS. 1A, 1B and 1C, a disc-shaped flat metal plate 1 was formed with cylindrical through-holes 2 therein according to a cutting or pressing process and thereafter the plate 1 was subjected to an etching treatment with an acid or the like, for example, at 30.degree.-50.degree. C. for about one minute and an oxidation treatment by being heated in an oxidizing atmosphere, e.g. air. Then, a lead 4 was secured and hermetically sealed to the hole 2 by softening or melting a sealing glass portion 3. After an oxide film formed on the surface of the plate 1 was etched by using HF, plating treatment (e.g. Ni or Au plating) was performed to provide the stem. Accordingly, the inner surface or inner wall of the hole 2 made in the flat metal plate 1, which was in contact with and was adhered to the glass portion 3 as shown in FIG. 1C, remained substantially unchanged during such etching treatment and oxidation treatment, i.e. remained substantially in the same state as that on the forming working. The contact surface was smooth as lower than 10 .mu.m of Rmax.
Accordingly, the length along the contact surface between the inner surface of the hole 2 made in the flat metal plate 1 and the glass portion 3, which extends from the upper surface A of the plate 1 to the lower surface B, is substantially equal to the linear distance between the upper and lower surfaces A and B.
Such a prior art stem for a semiconductor device has a problem that when a chip mounted on the stem is sealed with a cap by an electric resistance welding or the stem is fixed by a laser welding and the like, adhesion of the glass portion 3 to the inner surface of the hole 2 is loosened, resulting in a loss of airtightness. Namely, heat generated on a portion to be welded on welding is conducted to the joint area between the glass portion 3 and the flat metal plate 1, and then the adhesion of the glass portion 3 to the inner surface of the hole 2 is loosened by thermal stresses caused by a difference between the thermal expansion of the glass portion and that of the flat metal plate which are caused by the elevation in temperature.
The length along the joint area between the glass portion and the inner surface of the hole in the prior art is shorter than that in case where the contact surface has substantially irregularities, since the contact surface in the former is smooth or flat. The adhesion between the glass portion and the inner surface of the hole made in the flat metal plate is easily loosened and thus it is hard to obtain a high airtightness.